Conventionally, a Compton camera, which is employed as an apparatus for estimating the distribution of a nuclide gamma-ray source by measuring gamma-rays emitted from a target object with a gamma-ray detector disposed within the camera, is used to acquire a two-dimensional image of the gamma-ray source. In Compton cameras a physical collimator can be omitted since the path directions of gamma-rays can be estimated as Compton cones by measuring within the detector successive events of Compton scattering followed by photoelectric absorption. Another feature thereof is that simultaneous imaging of multiple nuclide radiation can be performed by using a semiconductor detector with a high energy resolution as the detector.
Up until now, Compton cameras have mainly been employed in space observation applications, however research has recently accelerated into application of Compton cameras to nuclear medical scanners. In space observation applications, since the purpose is to determine the path directions of gamma-rays, spatial resolution in the depth direction has not been a particular problem. However, when application is made to nuclear medical scanners, maintaining high precision for spatial resolution in all directions becomes necessary.
In a gamma-ray imaging apparatus provided with two divided-electrode type flat-plate germanium semiconductor detectors disposed parallel to each other, gamma-rays emitted from a gamma-ray source placed in front of the detectors are measured. There is a proposal (see, for example, Japanese Patent Application Laid-Open (JP-A) No. 2005-208057) for a gamma-ray imaging apparatus to measure events in which a gamma-ray is first incident to a front detector, undertakes Compton scattering, then the scattered gamma-ray is incident to the back detector where it is totally absorbed. Compton cones are then estimated for the measured events, and by measuring sets of two or more of these events, the position of the gamma-ray is determined as the position where cone surfaces overlap, and an image of the gamma emitting body is generated.